Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Fixed maturities, at amortized cost	$ 6,694,800,000	$ 6,372,700,000
Fixed maturities, allowance for credit losses	4,800,000	0
Equity securities, at cost	680,500,000	818,800,000
Premium receivables, allowance for credit losses	20,400,000	22,300,000
Debt, at fair value	$ 1,233,300,000	$ 820,200,000
Common stock, par value (in dollars per share)	$ 0.10	$ 0.10
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	65,406,442	66,665,888
Common stock, shares outstanding (in shares)	65,406,442	66,665,888